Antibiotic resistance plasmids encoding resistance to the clinically significant drug clindamycin (Cln), were used as model systems for the analysis of gene expression and the dissemination of genetic information in the pathogen Bacteroides fragilis. The Cln resistance determinant, ermFS, located within Tn4551 (on plasmid pBI136) was studied by gene cloning and DNA sequence analysis. The results were used for the generation of ermFS operon fusion vectors to study promoter activity and structure in IS4351. IS4351 bounds the composite transposons Tn4351 and Tn4551; studies indicate that transcription of ermFS is controlled by the IS elements. Data obtained with the operon fusion vectors located a significant promoter within the IS element that directed transcription toward ermFS in Tn4551. This location was verified by in vitro mutagenesis of the putative promoter followed by concomitant loss of ermFS activity in the fusion vector. Northern Blot analyses of ermFS tran-scripts confirmed these data but also revealed the presence of a second promoter within IS4351. The complete nucleotide sequence of IS4351 was determined and the results indicated that the element was 1155 bp in length with two 25 bp regions of imperfect dyad symmetry at its termini. The element was dominated by a large open reading frame which possessed weak amino acid homology to the transposase of IS3. Initial studies on the role of IS4351 in Tn4551 transposition and its regulation have involved the use of Northern Blot analysis. These results showed that plasmids bearing single copies of IS4351 had high levels of the putative transposase message. In the presence of two IS copies, transcription of the transposase was not detected suggesting the possibility that the regulation of Tn4551 transposition is at the level of transcription.